1. Technical Field
The present disclosure relates to the field of underwater acoustic (UWA) communications. More particularly, the present disclosure relates to detection, synchronization and/or online or real-time Doppler scale estimation of UWA communications.
2. Background Art
In general, underwater acoustic (UWA) communication (e.g., the sending and/or receiving of acoustic signals underwater) is a difficult and complex process. The unique characteristics of water as a propagation medium typically contributes to the problematic nature of UWA communication. For example, due to factors such as multi-path propagation and time variations of the channel, it is necessary to account for, inter alia, small available bandwidth and strong signal attenuation. Moreover, slow propagation speeds typically associated with acoustic signals may lead to significant Doppler shifts and spreading. Thus, UWA communication systems are often times limited by reverberation and time variability beyond the capability of receiver algorithms.
Unlike the development of wireless networks over radio channels, the development of underwater communication systems has generally occurred at a much slower pace. For operational reasons and due to concerns of battery life, it is typically important that underwater acoustic receivers be activated only by the presence of a signal to receive and demodulate. In general, when a signal is received, it is important that the receiver synchronize itself to the beginning of the message. Since relative transmitter/receiver motion can cause a dilation or compression of the message signal (i.e., Doppler), this time-scaling should be estimated and/or synchronized. Synchronization has typically entailed transmission of a known preamble prior to the data, which can be detected by the receiver. In general, existing preambles in underwater telemetry are almost exclusively based on linearly frequency modulated (LFM) signals, also known as Chirp signals. See, e.g., D. B. Kilfoyle and A. B. Baggeroer, “The state of the art in underwater acoustic telemetry,” IEEE Journal of Oceanic Engineering, Vol. 25, No. 1, pp. 4-27, January 2000.
LFM signals are commonly used because LFM signals typically have a desirable ambiguity function in both time and frequency—which matches well to the underwater channel and is characterized by its large Doppler spread. However, typical receiver algorithms are matched filter based, and attempt to synchronize a known template to the signal coming from one strong path and suppress other interfering paths. However, this approach suffers from at least the following two deficiencies: (1) the noise level at the receiver has to be constantly estimated to achieve a constant false alarm rate (CFAR) (usually accomplished using order statistics); and (2) performance will degrade in the presence of dense and unknown multipath channels.
Due to the slow propagation speed of acoustic waves, the compression or dilation effect on the time domain waveform must be considered explicitly. In general, once a Doppler scale estimate is obtained, a resampling procedure is typically applied before data demodulation. See, e.g., B. S. Sharif, J. Neasham, O. R. Hinton, and A. E. Adams, “A computationally efficient Doppler compensation system for underwater acoustic communications,” IEEE J. Ocean. Eng., vol. 25, no. 1, pp. 52-61, January 2000. One known method to estimate the Doppler scale is to use an LFM preamble and an LFM postamble around each data burst—e.g., so the receiver can estimate the change of the waveform duration. Unfortunately, this method typically estimates the average Doppler scale for the whole data burst and thus requires the whole data burst to be buffered before data demodulation—thereby preventing online real-time receiver processing.
Multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM) has prevailed in recent broadband wireless radio applications due to the low complexity of receivers required to deal with highly dispersive channels. For example, OFDM has been the workhorse modulation present in a number of practical broadband wireless systems (See, e.g., U.S. Pat. No. 5,732,113 to Schmidl et al. or T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Commun., vol. 45, no. 12, pp. 1613-1621, December 1997; H. Minn, V. Bhargava, and K. Letaief, “A robust timing and frequency synchronization for OFDM systems,” IEEE Trans. Wireless Commun., vol. 2, no. 4, pp. 822-839, July 2003.), notably wireless local area networks (IEEE 802.11a/g/n) (See, e.g., R. D. J. van Nee, G. A. Awater, M. Morikura, H. Takanashi, M. A. Webster, and K. W. Halford, “New high-rate wireless LAN standards,” IEEE Communications Magazine, vol. 37, no. 12, pp. 82-88, December 1999). The primary advantages of OFDM over single-carrier schemes is the ability to cope with severe channel conditions, e.g., frequency-selective fading due to multipath propagation without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. In general, a receiver can effectively correlate the received signal with a delayed version of itself, since, due to a cyclic prefix structure, the repetition pattern persists even in the presence of unknown multipath channels. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and substantially eliminate intersymbol interference (ISI). Channel equalization using OFDM is further simplified by approximating the effects of frequency-selective channel conditions as a constant for each OFDM sub-channel provided that each sub-channel is sufficiently narrow-band.
These advantages motivate the use of OFDM in underwater environments as well. However, as noted above, UWA channels are far more challenging than their radio counterparts. Specifically, synchronization algorithms that work in wireless radio channels will not perform well, if at all, in UWA channels with large waveform expansion or compression because the repetition period changes to an unknown value. Therefore, typical apparatus, systems and methods adapted for radio communication are inadequate for UWA communication because the necessary channel scaling associated with UWA transmissions is not accounted for—e.g., because such scaling difficulties are not present in radio transmissions. The problems associated with UWA transmission and reception are not present and therefore not considered by current radio communication methods, systems and apparatus. Further, many UWA transmission schemes which attempt to overcome the difficulties associated with UWA are examined by offline data processing based on recorded experimental data.
As a result, current UWA communication systems, methods and apparatus fail to adequately detect, synchronize and Doppler scale estimate UWA communication signals in such a manner that enables online or real-time receiver operation. These and other inefficiencies and opportunities for improvement are addressed and/or overcome by the apparatus, systems and methods of the present disclosure.